MacBook pro A2251 Boot failure

[yehuday]

Hi Everyone

I have a Macbook pro A2251 which fails to boot.
When i try to start it the fan momentarily spins full speed, then shuts off and it doesn't start.
This then repeats every few minutes. as if the board is stuck in a reboot loop

When i inspected the PCB i couldn't find any indication of liquid damage
I tried a few SMC reset sequence i found online but none helped (perhaps i did them wrong)

i am capable of opening the case and measuring voltage at test points. i also got the brd and layout files but i have no idea how to debug this issue
Any advice is appreciated.

Yehuda

[kevingill]

Go through the schematic and measure the main power rails voltages. Post here

[yehuday]

i couldn't really find the TP of the main power rails so i just measured a few TP with different voltage

here is my findings so far

PP5V_S5 - OK
PP3V3_S3 - OK
PP1V1_S3 - OK
PP1V8_S5 - OK
PP5v_main_vcore1/2/3 - OK
PP0v9_ssd0 - OK

PP0V9_TBT_X_LC - No voltage

[mon2]

Always post the measurements, not the interpretations of the values.

[yehuday]

This time with values

PP5V_S5 - 5.09v
PP3V3_S3 - 3.33v
PP1V1_S3 - 1.09v
PP1V8_S5 - 1.79v
PP5v_main_vcore1/2/3 - 5.09v
PP0v9_ssd0 - 0.89v

PP5V_S4 - 0V
PP0V9_TBT_X_LC - 0V

[mon2]

What is the voltage to ground of PPBUS_G3H at fuse F7000? Check both sides of the fuse. Is this voltage stable?

Can you confirm the netname that is 0v? Is it PP5V_S4SW? The full net name from the schematics should be posted to assist in quick searches.

Remove all power. Meter in resistance mode. Measure the resistance to ground of PP5V_S4SW. We are checking if there is a shorted part on this rail by checking the resistance to ground. One meter probe on PP5V_S4SW (anywhere on the board should be ok); other meter probe to ground (metal shield is ok).

No power / no battery to the board during these tests.

Usually on cases like yours, it is a good idea to test the resistance to ground of each inductor on the board. These are usually grey in color and will be with 2 leads. Each such inductor is linked to a local power rail. We are checking each power rail for possible shorted parts. So check the resistance in ohms and report any suspects. With a shorted part, the local power supply will sense this condition through the current sense leg of the power supply and power it down if out of range.

[yehuday]

thanks for the help

netname is indeed PP5V_S4SW.
PPBUS_G3H voltage at fuse F7000 moves between 12.1 to 12.5v on both sides. it is stable on 12.45 drops to 12.1v and then back up
i couldn't get a resistance measurements for PP5V_S4SW. it wasn't shortcut, it was outside the 200M range which is strange.

all the inductors i measured (total of 9) had a resistance to ground ranging from 40 ohm to 2k ohm. none were shortfused.
i suspect this coud be a stuck DFU issue but i can't get my hands on "apple configurator 2" dmg to test

[mon2]

apple configurator 2 - is available on the Apple store. Use your apple account to access the store. There is a requirement of the OS version to permit the use of this version. If you need an older version, found that too but through another link shared by a tech on Reddit I believe. Let me know if you need the link(s).

Be sure to use a quality USB cable.

[yehuday]

For some reason it doesn't exist on my app store only Apple configuration (not 2). Could be a country/ region thing
AFAIK it doesn't support macbook pro recovery

[mon2]

The A2251 is on the DFU list:

DFU Mode Restore (Macs) - LogiWiki

Try the download from here:

Download Apple Configurator 2 for Mojave : r/macsysadmin (reddit.com)

[yehuday]

i managed to install Apple configurator 2 and attached the macbook to another macbok air but no device was detected
i tried all USB-c port combinations but to no avail

back to square 1 i guess.
my only hint is ATM is PP5V_S4SW being 0 V
i will try i few other power plane pins and see what comes out

[mon2]

Remove all power.

• Check current sensing resistors: R7021, R7022, R7061, R7062.
  • Their values are all 1 ohm

[yehuday]

okay. i see they are all located in the top side of the board.
Is there an easy way to access it or should i remove all flex and screws and flip the board ?